All of your examples requited massive violence to destabilize their situations. Nazi Germany would likely not have happened if WWI did not happen. Same with China and Russia.
I'm also not saying violence is never the answer. Clearly violence was needed to stop the Nazis once they started.
But, one tool gives you the ability to avoid violence, while the other almost always leads to it.
But massive, destabilising violence has been a constant of human existence (I'm still listening to this history podcast, and, my God, the Roman Empire was not a terribly stable institution!).
It's not clear to me we really disagree though. I just think people can overestimate the power of 'talking' and persuasion. I mean, ultimately, even as we talk, we rely on 'the police' to maintain the conditions for talking - and policing is always in the end dependent on the threat of violence. This killer was eventually apprehended via violence (running his car off the road, as I understand it).
I certainly don't have any ultimate answers to this kind of problem. I do think the most banal solution of 'restricting guns more' would have helped a lot though. I find it tragic and disappointing that NZ didn't impose the same restrictions Australia did after Port Arthur. Did they think they were surrounded by a magic field of protection that would prevent it happening there?
It seems to be always the way - countries only do the absolute bare minimum to make less likely only the exact same massacre that just happened [the exception being the US, which won't even do that much]. Same happened in the UK - after Hungerford they only banned the semi-auto rifles that were used in that instance, and it only dawned on people that handguns can be just as deadly after Dunblane. Dunblane could have been averted if legislators had shown a bit more imagination post-Hungerford. Even after Dunblane I'm not convinced we went far enough.
Maybe you can't solve the evil in people's hearts and heads, but you can make it harder for them to act on it.